We have attached fluorescent probes to the lac repressor of Escherichia coli. N-(Iodoacetylaminoethyl)-1-naphthylamine-5-sulfonate has been used to selectively and covalently label sulfhydryl groups. Both 1-anilino-8-naphthalene sulfonate and bis-4,4'-(1-anilino-8-naphthalene sulfonate) bind noncovalently to this protein. These probes have been used to detect conformational changes occurring in this protein when it binds to the nonspecific DNA, poly(d(A-T)). The rapid kinetics of these conformational changes have been monitored using stopped-flow techniques. The proposed work will extend these studies to operator DNA. It is hoped that a comparison of these results with those obtained using nonoperator DNA will help to explain the repressor's specificity for operator DNA. 1-Anilino-8-naphthalene sulfonate has been used to measure the binding site size of the repressor on poly(d(A-T)). This size, 33 plus or minus 4 base pairs per tetramer, differs from that reported by other investigators. In order to clearly interpret the kinetic results obtained with nonoperator DNA, the stoichiometry of repressor-poly(d(A-T)) association must be known. A variety of experimental approaches will be employed to provide this information. The selective modificaton of residues within the amino terminal region of the lac repressor with fluorescent probes will be attempted.